On Tue, Sep 15 2009, Jan Kara wrote:> On Mon 14-09-09 20:36:54, Jens Axboe wrote:> > On Mon, Sep 14 2009, Trond Myklebust wrote:> > > On Mon, 2009-09-14 at 15:02 +0200, Jan Kara wrote:> > > > On Mon 14-09-09 11:36:29, Jens Axboe wrote:> > > > > We do this automatically in get_sb_bdev() from the set_bdev_super()> > > > > callback. Filesystems that have their own private backing_dev_info> > > > > must assign that in ->fill_super().> > > > > > > > > > Note that ->s_bdi assignment is required for proper writeback!> > > > > > > > > > Acked-by: Christoph Hellwig <hch@infradead.org>> > > > > Signed-off-by: Jens Axboe <jens.axboe@oracle.com>> > > > Hmm, looking at this again, I'm not sure this will work for NFS. It seems> > > > to set mapping->backing_dev_info to its private backing dev info for> > > > regular files while it leaves it intact for other inodes (e.g.> > > > directories). I'm not sure why it does so but it seems its inodes end up on> > > > two different BDI lists and thus they wouldn't be synced properly. Trond,> > > > do I read the code properly?> > > > Also we definitely need to set *some* bdi in nfs_get_sb as otherwise sync> > > > won't work for it.> > > > > > There hasn't really been a need for a bdi in NFS other than for the> > > regular file read and writeback code. The main reason for making it> > > private was to ensure that we could set a per-superblock readahead limit> > > that was a decent multiple of the server's preferred read block size.> > > > > > Is there any reason why we couldn't set sb->s_bdi to point to that> > > private bdi?> > > > No, that should work fine. NFS already works fine with the bdi flusher> > threads, so you should just point it at that bdi.> But will it really work well? I mean if we sync the superblock on the> client, it will sync only the private BDI. So it won't sync any directory> inodes because they are on the default_backing_dev_info (NFS leaves> sb->s_bdev at NULL).

No you are right, it wont be complete. I talked to Chris about thisyesterday, and we agree that the best option is to give NFS the 'btrfstreatment' bdi wise.